Fatty Acids and Stomach Contents Reveal Dietary Overlaps of Two Prospering Squid Species in the North Sea

Worldwide, the distribution ranges of cephalopods are expanding due to climate change. In the North Sea, the broadtail shortfin squid <i>Illex coindetii</i> (Ommastrephidae) has recently established a successful breeding population and now coexists with the local veined squid <i>Loligo forbesii</i> (Loliginidae). Among other squids, both species are potential food competitors. To investigate the trophic preferences of either species, we analyzed the fatty acid (FA) composition of the mantle tissue and the digestive gland (DG) via gas chromatography (GC). According to the FA trophic marker concept, FA signatures may be indicative of the staple food over a longer period. Additionally, we investigated the stomach contents of both species visually to obtain information about their recently consumed food. The FA patterns of the DG of both species suggested moderate overlaps of the prey spectra. The stomach contents of <i>I. coindetii</i> and <i>L. forbesii</i> showed significant overlaps, further confirmed by the trophic index Pianka's niche overlap. Both squid species fed on commercially important taxa, including herring, cod, and decapods. The population of <i>I. coindetii</i> in the North Sea will rise along with warming. As a consequence, competition with <i>L. forbesii</i> for food will increase, and food web interactions will change. Predation between both squid species may increase as well.